1. Field of the Invention
The invention relates generally to a sample preparation membrane separation modules and methods and more specifically to single-pass tangential flow filtration operation for the concentration of liquid samples.
2. Description of the Related Art
Ultrafiltration (UF) and microfiltration (MF) membranes have become essential to the separation and purification in the manufacture and research of biomolecules. Biomolecular manufacturing and laboratory sample preparation, regardless of the scale, generally employs one or more processing steps including filtration (UF or MF). The attractiveness of these membrane separations rests on several features: including, for example, high separation power, and simplicity (e.g., requiring only the application of a pressure differential between feed and permeate). This simple and reliable one-stage “cut” of the sample into two fractions makes membrane separation processes a valuable approach to separation and purification.
In one class of membrane separations, the species of interest is that which is retained by the membrane, and the objective of the separation is typically to remove smaller contaminants, to concentrate the solution, or to affect a buffer exchange. In another class of membrane separations, the species of interest is that which permeates, and the objective is typically to remove larger contaminants. In MF, the retained species are typically particulates, organelles, bacteria or other microorganisms, while those that permeate are proteins, colloids, peptides, small molecules and ions. In UF the retained species are typically proteins and, in general, macromolecules, while those that permeate are peptides, ions and, in general, small molecules.
The ability to maintain a reasonably high flux is essential in the practice of membrane processes. Low flux can result in long filtration times or large modules, resulting in increased cost and large hold-up volumes (i.e., the volume including the retained species remaining in the module). The filtration process itself induces the creation of a highly concentrated layer of the retained species on the surface of the membrane, a phenomenon called “concentration polarization” (or simply “polarization”), which reduces the flux from an initial value obtained immediately at the start of filtration. In the absence of counter measures the accumulation of “polarized” particles or solutes results in vanishingly small fluxes and bringing the processes to a stand-still. One conventional approach to overcoming the effects of concentration polarization in the practice of ultrafiltration is to run the separation in “tangential flow filtration” (TFF) mode.
TFF modules are devices having flow channels formed by the membrane through which the feed stream flows tangentially to the surface of the membrane. The tangential flow induces a sweeping action that removes the retained species and prevents excessive accumulation, thereby maintaining a high and stable flux. Because higher tangential velocities produce higher fluxes, the conventional practice of TFF calls for the use of high velocities in the flow channels, which in turn result in very high feed rates. These high feed rates result in low conversion, typically less than 10% and often less than 5%. Low conversion means that the bulk of the feed stream exits the module as retentate without having been separated.
One commercially important area for UF separations and purification is at the preparation of analytical samples (e.g., sample volumes less than 100 ml). The application of conventional TFF processes to sample preparation at the analytical scale is generally believed not to be practical due to the complications inherent in the use of pumps and recirculation loops. As a result, UF separations at these scales are practiced almost exclusively in a “dead-ended” mode, resulting in an inherently low flux due to concentration polarization. Centrifugal UF devices have been developed for this scale to mitigate the low flux of dead-ended UF separations. However, while these have become the dominant format for analytical scale UF, they typically require centrifuges capable of exposing the UF device to accelerations as high as 14,000 g. Furthermore, in spite of these accelerations, many separations still require a long time, as high as one hour. Finally, the recovery of the retentate presents special difficulties in these approaches since it may be spread as a thin film over the surface of the membrane.
One prior art device disclosed in U.S. Pat. No. 4,761,230, Pacheco, et al., includes first and second housing sections with a flow channel extending therebetween. A membrane filter forms one boundary of the flow channel. A pair of reservoirs, one for feed and the other for permeate collection, are integrally formed with the first housing section. A fluid communication path is established from the first section to the second section and then through means of a deformable chamber to the flow channel. The deformable chamber is adjacent to a rigid surface that is integral with one of the housing sections and in this manner is adapted to pump fluid through the system when interfacing with a pump. This device also operates in a continuous recirculation mode during concentration of batch samples and includes a recirculation loop.
U.S. Pat. Nos. 6,692,702, Burshteyn, et al. and 6,692,968, Burshteyn, et al, teach a method for utilizing a filtration device for removing interferants from a sample containing cells in an automated apparatus is disclosed. The filtration device includes a microporous hollow fiber membrane having a plurality of pores sized to retain cells while allowing smaller diameter interferants to pass through the membrane. The apparatus also includes a means for moving the sample from a sample container to and from the filtration device. The disclosed method utilizes a vacuum source to aspirate the sample into a lumen of the hollow fiber membrane so that the sample is retained in the lumen space until expelled into an analysis container or transported to an analyzer.
None of the prior art devices and methods provides rapid, controlled conversion without the use of numerous venting valves, recirculation loops and pumps in addition to simple construction and operation. Thus, the need exists for devices and processes suited for sample preparation in life science and diagnostics laboratories which are able to yield high reliable flux and high conversion without the need of recirculation loops, numerous valves and intermediate pumps, and that can be readily driven by the low-pressure differentials and which are simple to control. It would also be desirable to operate a bio-processing separation at the sample preparation scale in a single-pass mode while providing a high conversion with a relatively low hold up volume and effective recovery of the separation products.